1. Field of the Invention
The present invention relates to a recording medium rotation control apparatus for an information reproducing apparatus for reproducing information from a recording medium in which information modulated in a run length limited code has been recorded on concentric or spiral information tracks at a constant linear velocity.
2. Description of the Prior Art
In some recording media, information modulated in the run length limited code is recorded on concentric or spiral information tracks at a constant linear velocity in order to increase recording density as shown, for example, in U.S. Pat. Nos. 4,397,011 and 4,466,089. The compact disk is a typical example of such recording media. When information is to be reproduced from a recording medium of this type, it is necessary to maintain the linear velocity of the recording medium constant by changing the rotation velocity thereof according to the position of an information track from which information is to be reproduced.
In order to accomplish this, there is available a method for controlling the rotation of the recording medium to be a constant linear velocity by using a signal reproduced from the recording medium as disclosed, for example, in Japanese Laid-Open Patent Application No. 57-58269. This method utilizes the principle that a maximum or minimum transition interval time from one state inversion ("0" to "1" or "1" to "0") to the next state inversion of a signal reproduced from the recording medium varies with the linear velocity during reproduction. Namely, such maximum or minimum transition interval is compared with a reference value of maximum or minimum transition interval which is determined in dependance upon the modulating method, thus achieving rotation control for the constant linear velocity.
This method, however, has a defect in that control errors would easily occur due to the an erroneous deviation of transition interval or of the detected maximum or minimum transition interval. Such an erroneous deviation of the transition interval is caused by the asymmetry of the pulse waveform of the reproduced signal as shown in U.S. Pat. Nos. 4,397,011 or No. 4,466,098 or by a deviation of a threshold level for reshaping the pulse waveform of the reproduced signal. This method further has a defect in that the rotation control would be easily disturbed since the maximum or minimum transition interval undergoes a great change when the reproducing means moves across an information track of the recording medium or when the linear velocity is so small that the reproducing means cannot accurately track the information tracks of the recording medium.